UN Security Council members have voiced fears about the direction of violence-wracked Syria as the rebel Free Syrian Army gave the government of President Bashar al-Assad a 48-hour deadline to observe international envoy Kofi Annan's peace plan.
The FSA's Colonel Qassim Saadeddine said in a video published online on Wednesday that the government must "implement an immediate ceasefire, withdraw its troops, tanks and artillery from Syrian cities and villages".
Council members were briefed by Jean Marie Guehenno, Annan's deputy, on Wednesday as 13 bodies were discovered in the east of the country with their hands tied behind their backs and signs that some had been shot in the head from close range.
Colonel Qassim Saadeddine said if there was no response by Friday lunchtime, the FSA would consider itself "no longer bound by the.. peace plan".
"It [the government] should also allow immediate humanitarian aid to all affected areas and free all detainees... The regime should also enter into a real and serious negotiation through the United Nations to hand over power to the Syrian people," he went on.
The commander of the group, Riad al-Asaad, however dismissed the deadline.
"There is no deadline, but we want Kofi Annan to issue a declaration announcing the failure of this plan so that we would be free to carry out any military operation against the regime," he told Al Jazeera on Thursday.
The comments came as the 15-nation Security Council was briefed about the worsening crisis and the peace efforts by Annan.
"There had been very few positive elements in Syria," Russian ambassador Vitaly Churkin said following the closed-door briefing.
Churkin called the political situation in Syria "sad" because the process to bring both sides in the conflict into negotiations had so far failed to get off the ground. He complained that "nobody is implementing" Annan's six-point peace plan, including last month's ineffective truce.
US hints at bypassing UN
US ambassador Susan Rice said the unabated violence and failure to implement Annan's plan could lead to the "worst scenario" in Syria, and that the pessimistic scenario is the "most probable."
"The decision rests on the Damascus side," she said. "The council would have to act swiftly and responsibly in order to avoid such a scenario and support the Kofi Annan plan."
Rice said there were three ways the Syrian conflict could end.
The first would be if Assad's government decided to comply with its obligations under Annan's six-point peace plan - stopping its military assaults on Syrian towns, withdrawing heavy weapons, returning troops to barracks and talking with the opposition on a "political transition."
In-depth coverage of escalating violence across Syria
The second option would involve the council taking action to pressure Damascus to fully comply with the Annan plan, she said.
"In the absence of either of those two scenarios there seems to be only one other alternative, and that is indeed the worst case," Rice said, adding that it was unfortunately looking like "the most probable."
"That is that the violence escalates, the conflict spreads and intensifies," she said. "It involves countries in the region, it takes on increasingly sectarian forms, and we have a major crisis not only in Syria but in the region."
In such a case, Rice said, the Annan plan would be dead and the Syrian violence would become "a proxy conflict with arms flowing in from all sides".
"And members of this council and members of the international community are left with the option only of having to consider whether they're prepared to take actions outside of the Annan plan and the authority of this council," she said.
She did not specify what kind of "actions" she meant.
More mass killings
Also on Wednesday, Major-General Robert Mood, the head of the UN observers in Syria, said 13 bodies had been discovered in the east of the country, with their hands tied behind their backs and signs that some had been shot in the head from close range.
The bodies were discovered late on Tuesday in the area of Assukar, 50km east of Deir al-Zor. It comes after the weekend massacre of more than 108 people in Houla.
Survivors blamed pro-government armed men for at least some of the carnage in Houla. The Syrian government denied its troops were behind the killings and blamed "armed terrorists" and said it would conclude its own investigation into the deaths by Wednesday, but it was not clear if the findings would be made public.
The UN's top human rights body plans to hold a special session on Friday to address the massacre.
Wednesday's UN observer report underlined how a peace plan drafted by Annan has failed to stem bloodshed or bring Syria's government and opposition to the negotiating table.
Guehenno, Annan's deputy, told the Security Council that Syria's protesters "have lost fear and are unlikely to stop their movement", according to a diplomat with knowledge of the closed session.
Guehenno said direct engagement between government and opposition was "impossible at the moment" and expressed "serious doubts over the commitment of Syrian authorities to the Annan plan", the diplomat said.